This study provides a comprehensive attack on the structure and function of an important cell organelle, the cilium, centering around ultrastructural correlates of ciliary movement. An important facet has been the development of the sliding microtubule model of ciliary motion, which is presently being confirmed and enlarged. It is known that: (1) The 9 peripheral doublet microtubules of the axoneme slide relative to one another without contraction during bend production. (2) Sliding is generated by interaction of dynein ATPase arms of one doublet and sites, probably on the adjacent doublet, accompanied by ATP dephosphorylation. We are examining the hypothesis that: (2) The radial spokes attached to each doublet are a central part of the mechanism by which sliding is converted into bending. Changes in spoke configuration in bent vs, straight regions of the axoneme have been observed, within the ciliary membrane in ciliary growth and function.